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Indoor Environment and Energy Efficiency in Schools Part 1 Principles Francesca R. d’Ambrosio Alfano (ed.) Laura Bellia Atze Boerstra Froukje van Dijken Elvira Ianniello Gino Lopardo Francesco Minichiello Piercarlo Romagnoni Manuel Carlos Gameiro da Silva

rehva Federation of European Heating, Ventilation and Air-conditioning Associations

GUIDEBOOK

NO 13

REHVA

Indoor Environment and Energy Efficiency in Schools Part 1 Principles

Francesca R. d’Ambrosio Alfano (ed.) Laura Bellia Atze Boerstra Froukje van Dijken Elvira Ianniello Gino Lopardo Francesco Minichiello Piercarlo Romagnoni Manuel Carlos Gameiro da Silva

DISCLAIMER This Guidebook is the result of the REHVA volunteers. It has been written with care, using the best available information and the soundest judgment possible. REHVA and the REHVA volunteers, who contributed to this Guidebook, make no representation or warranty, express or implied, concerning the completeness, accuracy, or applicability of the information contained in the Guidebook. No liability of any kind shall be assumed by REHVA or the authors of this Guidebook as a result of reliance on any information contained in this document. The user shall assume the entire risk of the use of any and all information in this Guidebook. --------------------------------------------------------------------------------------------------------Copyright © 2010 by REHVA, Federation of European Heating, Ventilation and Air–conditioning Associations All rights reserved No part of this publication may be reproduced or transmitted in any form or by any means, electronically or mechanical, including photocopy recording, or any information storage and retrieval system, without permission in writing from the publisher. Requests for permission to make copies of any part of the work should be addressed to REHVA Office, 40 Rue Washington, 1050 Brussels – Belgium e-mail: [email protected] ISBN 978-2-930521-03-9 Printed in Turkey by Özgün Offset ii

List of contents

1

2

3

4

ENVIRONMENTAL COMFORT AND ENERGY SUSTAINABILITY .......... 1 1.1

Introduction.................................................................................................... 1

1.2

EU attention to energy saving in residential buildings.................................... 1

INDOOR ENVIRONMENTAL COMFORT ASPECTS..................................... 4 2.1

The Indoor Environmental Quality (IEQ)....................................................... 4

2.2

The importance of a good IEQ in schools....................................................... 6

2.3

Thermal comfort............................................................................................. 9

2.4

Indoor Air Quality........................................................................................ 18

2.5

Acoustic comfort .......................................................................................... 33

2.6

Visual comfort.............................................................................................. 39

ENERGY SAVING AND GLOBAL COMFORT .............................................. 53 3.1

Introduction.................................................................................................. 53

3.2

Impact of energy saving choices on global comfort...................................... 53

3.3

The Standard EN 15251 ............................................................................... 54

HVAC SYSTEMS ................................................................................................ 62 4.1

General considerations and typical school areas........................................... 62

4.2

Design criteria .............................................................................................. 63

4.3

Thermal loads............................................................................................... 67

4.4

HVAC equipment and systems in school buildings: typologies and selection criteria ........................................................................................... 68

4.5

HVAC equipment and systems in new school buildings............................... 73

4.6

HVAC systems in existing school buildings: typologies and retrofit criteria 79

4.7

Dehumidification problems at summer part load conditions......................... 79

4.8

Automatic control system and energy conservation criteria for HVAC systems in school buildings .............................................................. 80 iii

4.9

Central heating/cooling stations....................................................................... 82

4.10 Maintenance ................................................................................................. 83 5

6

ENERGY CONSUMPTION................................................................................ 85 5.1

Introduction.................................................................................................. 85

5.2

Energy consumption assessment methodologies for new buildings .............. 86

5.3

Energy consumptions and energy assessment benchmarking methodologies for existing buildings.................................................................................... 90

5.4

Primary energy evaluation............................................................................ 93

5.5

Energy considerations about schools ............................................................ 94

CASE STUDIES................................................................................................... 97 Case I – The assessment of IEQ of an Italian school ....................................................................97 Case II – A renovated school building with natural air supply and mechanical exhaust...........103 Case III – A renovated classroom with a displacement ventilation system................................108 Case IV – Poikkilaakso School Helsinki, Finland ......................................................................113

7

REFERENCES................................................................................................... 118

iv

REHVA – Federation of European Heating, Ventilation and Air–Conditioning Associations

REHVA, now almost 50 years old, is an organization of European professionals in the field of building services (heating, ventilation and air-conditioning). REHVA represents more than 100 000 experts from 29 European countries.

would have a good long term return of investment due to improved learning results and lower health care expenses. This guidebook gives a sound background on the basics of the human requirements with respect to indoor environmental conditions for the design and operation of schools for the optimal performance of the students. The book also describes the principles of good HVAC systems for schools. It focuses particularly on energy efficient systems for a healthy indoor environment. The book gives also practical guidance for selection, installation and operation of HVAC systems. With case studies from different climatic conditions it illustrates how the renovation can impact on energy performance and indoor environment of school buildings.

REHVA’s main activity is to develop and disseminate economical, energy efficient and healthy technology for the mechanical services of buildings. The work is supervised by the Board of Directors. REHVA Guidebook projects are coordinated by the Technology and Research Committee of REHVA. Several task forces are currently working on REHVA Guidebooks such as: Indoor environment in museums, Indoor environmental investigations, New air distribution systems, Radiant heating, Low energy cooling and many others.

This guidebook is a project of several member countries of REHVA. The work has been coordinated by REHVA´s Italian member AICARR.

School buildings are very important for society. The number of children in schools in REHVA countries throughout Europe is more than 100 million, a considerable part of population. School buildings also represent a significant part of the building stock, and also a noteworthy part of the total energy use. Unfortunately studies in many countries have shown numerous shortcomings in the indoor environment of schools. Actually, children in schools are more sensitive to environmental exposure than adults. Society should do more to protect them from harmful exposure in schools. All students and employees in schools have the right to a healthy indoor environment. Investment in the quality of school buildings

The REHVA Board would like to express its sincere gratitude to AICARR and the working group for their invaluable work. REHVA would also like to express its gratitude to the universities, institutions and companies that supported the work by allowing and encouraging their experts to participate in the production of this guidebook. Olli Seppänen Secretary General of REHVA v

Member countries of REHVA Belgium Bosnia Bulgaria Croatia Czech Republic Denmark Estonia Finland France Germany

Hungary Ireland Italy Latvia Lithuania The Netherlands Norway Poland Portugal Romania

Russia Serbia Slovakia Slovenia Spain Sweden Switzerland Turkey United Kingdom

Working group This book is developed with a working group consisting of the following experts: • • • • • • • • • • • •

Laura Bellia, Professor, University of Napoli Federico II, Italy Atze Boerstra, Consultant, BBA Indoor Environmental Consultancy, The Netherlands Francesca R. d’Ambrosio Alfano, Professor, University of Salerno, Italy Manuel Carlos Gameiro da Silva, Professor, University of Coimbra, Portugal Elvira Ianniello, PhD, University of Salerno, Italy Jarek Kurnitski, Adjunct Professor, D.Sc., Sitra, Finnish Innovation Fund, Finland Gino Lopardo, PhD student, University of Salerno, Italy Francesco Minichiello, Professor, University of Napoli Federico II, Italy Jari Palonen, Technical University of Technology, Helsinki, Finland Piercarlo Romagnoni, Professor, University IUAV Venezia, Italy Dennis Schuiling, Consultant, Royal BAM Group, BAM techniek, The Netherlands Froukje van Dijken, Consultant, BBA Indoor Environmental Consultancy, The Netherlands

Reviewers The following experts have reviewed the book and made valuable suggestions for improvements. • Derrick Braham, Consult, UK • Dusan Petras, Professor, Slovakia • Peter Stankov, Professor, Bulgaria

vi

1 ENVIRONMENTAL COMFORT AND ENERGY SUSTAINABILITY 1.1

Introduction

Building construction and renovation design is assuming a more and more human oriented philosophy in the last years. Building materials, systems and components become more and more advanced and sophisticated in order to meet this change. This happens because of changing needs of people with respect to the indoor environment, where they spend the most part of the day and that should be as comfortable as possible, in terms of indoor environmental quality, IEQ, which is a measure of the whole of thermal, visual, acoustic comfort and acceptable indoor air quality conditions in an environment. It is clear that safety is the most important aspect in building construction/renovation design, but it should be always kept in mind that the final aim of designers is to let people stay healthy in the indoor environment they construct.

past, in order to meet requirements of safety, energy saving, maintainability, sustainability and occupants comfort. Anyway, design is not the only aspect to be taken into consideration, because the operation of the building and the conduction and maintenance of systems are as important as the design phase. In fact energy efficiency and comfort are strictly related to how the building and its devices are used. For example, even if a potentially very efficient system is designed and realized, bad use or maintenance could cause unforeseen energy losses. An integrated approach in the design of school buildings and systems is required to meet all mentioned needs, because a high number of variables and their relationships, in many cases very complex ones, have to be considered (d’Ambrosio Alfano et al., 2006). 1.2

EU attention to energy saving in residential buildings

An important factor, that should not be neglected, is the growing consciousness of researchers concerning the effects of a poor indoor environment on people. Focusing only on unhealthy diseases, let us think of the loss of concentration in noisy environments or of the effects on performance and productivity due to poor indoor air quality.

The Directive 2002/91/EC of the European Parliament and of the Council on 16 December 2002 on the energy performance of buildings raises some fundamental questions connected with the increasing requirement of energy, and of the energy crises during the last few years.

Design, techniques, materials rules and standards are developing in this sense, so, when a new building is designed, or an existing one is renovated, more aspects have to be considered with respect to the

One of the most relevant aspects stated in the Directive introduction, is the fact that the residential and tertiary sectors, the major part of which are buildings, accounts for more than 40% of final energy 1

REHVA Indoor Environment and Energy Efficiency in Schools Guidebook – Part 1

consumption in the Community and is expanding, a trend which is bound to increase its energy consumption and hence also its carbon dioxide emissions (European Union, 2002). That is why the European Community considered it necessary to call to the attention of Member States the fact that demand management of energy is an important tool enable the Community to influence the global energy market and hence the security of energy supply in the medium and long term (European Union, 2002).

Member States are required to get equipped with calculation methodologies of energy performance of buildings for design and verification of minimum performance requirements. Methodologies must be established on a national or regional scale.

The Directive 2002/91/EC objective is to promote the improvement of the energy performance of buildings within the Community, taking into account outdoor climatic and local conditions, as well as indoor climate requirements and cost effectiveness.

An important aspect, often remarked in the Directive text, concerns the influence that such choices concerning energy saving can have upon environmental quality of buildings. At point 8 of introductive considerations, it can be read that systems should be designed in such a way that the amount of energy required in use will be low, having regard to the climatic conditions of the location and the occupants (European Union, 2002).

The Directive lays down requirements concerning: • general framework for a methodology of calculation of the integrated energy performance of buildings; • application of minimum requirements on the energy performance of new buildings; • application of minimum requirements on the energy performance of large existing buildings that are subject to major renovation; • energy certification of buildings; • regular inspection of boilers and of air conditioning systems in buildings and, in addition, an assessment of the heating installation condition in which the boilers are more than fifteen years old.

2

All new buildings must satisfy minimal performance standards; furthermore all existing buildings, having a useful area over 1000 m² and subject to major renovation, should be refurbished so that the minimum requirements are satisfied.

In other parts of the Directive text references to people comfort, in particular, at point (16): Public activity buildings and buildings frequently visited by public should set an example by taking environmental and energy considerations into account….Moreover the displaying of officially recommended temperatures, together with the actual measured temperature, should discourage the misuse of heating, air conditioning and ventilation systems. This should contribute to avoiding unnecessary use of energy and of safeguarding comfortable indoor climatic conditions (thermal comfort) in relation to the outside temperature (European Union, 2002).

1 ENVIRONMENTAL COMFORT AND ENERGY SUSTAINABILITY

An important element, introduced by Directive 2002/91/EC is the energy performance certificate of a building: Member States shall censure that, when buildings are constructed, sold or rent out, an energy performance certificate is made available to the owner to the prospective buyer or tenant, as the case might be. The validity of the certificate shall not exceed 10 years (European Union, 2002).

position and orientation of building, passive solar systems and solar protections, natural ventilation, indoor climatic conditions, including outdoor climate.

Energy certification aims to determine and certify energy consumption or need of a building, to classify a building with reference to its energy consumption and to also indicate ameliorative measures.

The energy performance certificate of a building should include reference values such as current legal standards and benchmarks in order to make it possible to compare and assess the energy performance of the building. The certificate should be accompanied by recommendations for the cost-effective improvement of the energy performance.

The general frame and calculation methodology are indicated in the annex to the Directive. In particular, calculation methodology should contain at least the following aspects: thermal characteristics of the building, heating installation and hot water supply, air conditioning installation, ventilation, lighting installation,

Calculation should consider the advantages related to the use of renewable energy sources and natural daylight. Furthermore, buildings should be adequately classified into categories.

Finally, Directive requires Member States to legislate for measurements to assure regular inspection of boilers and air conditioning installations.

3

2 INDOOR ENVIRONMENTAL COMFORT ASPECTS 2.1

The Indoor Environmental Quality (IEQ)

Indoor environmental quality is related to the coexistence of thermal comfort, visual comfort, indoor air quality and acoustic comfort. In these last few years more and more interest on global comfort in life and work places is observed. Several studies have proved that if someone lives in a comfortable environment, his work performances and productivity improve (Andersson et al., 2006). Furthermore indoor environmental factors affect the energy consumption in a building and so, interest in assessment and design of these factors is increasing for this reason as well.

The quality of the indoor environment is dependent on many factors. These factors may be subdivided into the following categories: • • • •

outdoor conditions; building; building services; human activities.

It may be seen in Figure 2.1 that a good indoor environment is realised through good design of building and systems. Adequate management and maintenance and a considered interior are also important, as is the responsible use of the facilities by the building users.

Figure 2.1 Factors which influence the Indoor Environmental Quality.

4

2 INDOOR ENVIRONMENTAL COMFORT ASPECTS

For indoor environmental problems in existing as well as in new or renovated buildings, account must be taken of the multifaceted nature of the indoor environment.

In the short term, the thermal climate may have consequential health effects. Examples of health problems which may be related to this are:

The quality of the indoor environment mainly affects the well-being and comfort of the building users.

• headache (for example associated to low temperatures and high humidities (Bianchi et al., 2003)); • fatigue; • dizziness; • motor system detriment (for low temperatures).

Besides this, a poor indoor environment may lead to various health problems, which may be subdivided depending on whether they have short-term or longterm effects on health.

2.1.2 Indoor Air Quality

Short-term effects arise after a single exposure or repeated exposures, mostly within 24 hours (or at most a few days) after exposure. These short-term effects are mostly of short duration and reversible. The complaint may often be relieved immediately by removing the cause (source).

Exposure to poor indoor air quality may immediately result in olfactory discomfort (unpleasant smells), and complaints about stuffy or stale air. In the short term, poor indoor air quality may also lead to complaints about dry air (irritation of the mucous membranes).

Comfort and health complaints cause an increase in sick leave and a reduction in productivity and learning performance. This is of course associated with increased costs.

Poor indoor air quality may also affect health in the short term. Examples of short-term effects are:

In the following some comments about effects of poor environment on people are reported. 2.1.1 Thermal comfort Thermal discomfort may be considered; complaints occur about: • • • • • •

high temperatures; low temperatures; varying temperatures; draughts; radiation; hot or cold feet (floors).

• eye irritation / red eyes; • complaints about dry throat / throat irritation; • blocked or running nose; • headache; • unusual fatigue (particularly at the end of the day); • dizziness. Serious health problems in the short and medium term are: • Incidence of infections, such as flu or colds; • Asthma attacks and sensitisation of persons with a genetic tendency to asthma and allergens; 5

REHVA Indoor Environment and Energy Efficiency in Schools Guidebook – Part 1

• Infection with Legionella bacteria (Legionnaire's disease) as a result of exposure to aerosols (tiny water droplets) infected with Legionella bacteria in the air, e.g. in showers in gyms; • Carbon monoxide poisoning (symptoms are e.g. persistent headache and drowsiness). This seldom happens in educational buildings. 2.1.3 Acoustic comfort Poor acoustics in rooms and noise from other rooms, from building services or from outside (traffic, playground) may lead to noise nuisance. Noise nuisance may lead to: • reduction in concentration; • reduction in speech intelligibility; • voice problems. 2.1.4 Visual comfort Good lighting (by both natural and artificial light) ensures that both the blackboard and assignments may be read more easily and comfortably. Visual discomfort may reveal itself in the following complaints: • too little daylight or artificial light; • dazzling daylight or artificial light; • inadequate visibility. Light also has an influence on health. The biological clock is influenced by light, so that physiological processes are regulated, such as the sleep-wake rhythm, body temperature and alertness. Poor lighting may also contribute for example to: • eye irritations; • neck and shoulder problems; • fatigue and headache. 6

2.2

The importance of a good IEQ in schools

IEQ is very important in school environments. Children particularly are extra sensitive to a poor indoor environment. Children are physically still developing (their lungs are not full-grown, for example) and in comparison to (healthy) adults will suffer the consequences of a poor indoor environment earlier. Various investigations have shown that the indoor air quality in many schools worldwide is poor (Daisey et al., 2003; van Dijken et al., 2006). Schools are occupied by large numbers of pupils. These pupils all produce pollutants such as CO2, moisture, bioeffluents and dust. Moreover, building components, furnishings and equipment contribute to the release of indoor air pollutants such as microbial contaminants, volatile organic compounds (VOCs), formaldehyde and plasticisers. Adequate ventilation is needed to remove these contaminants from indoor air. Apart from pollutants, pupils also produce heat, which, without proper ventilation, increases the temperature in the classroom. It has been shown that thermal environment can affect students not only concerning comfort but even concerning health (Bianchi et al., 2003) and school performance (Mendell and Heath, 2005). For these reasons, the correct design of the building envelope and its systems is fundamental to avoid a poor and unpleasant thermal environment. A good acoustic environment in school is also an important factor, since it is related to the main tasks of students: hearing,

2 INDOOR ENVIRONMENTAL COMFORT ASPECTS

understanding and learning. Inadequate intelligibility and too high background noise can be cause of diminished performance and health problems. Poor light environment can affect many tasks of students at school and, consequently, their performance. When aiming to obtain visual comfort and effective visual tasks performance it is important to pay attention to the design of lighting systems and to the choice of transparent surfaces distribution and size. 2.2.1 Indoor environment, asthma and allergens The number of children with air-qualityrelated health problems is often underestimated. Worldwide, asthma is the most prevalent chronic illness in children. It has been shown by the ISAAC investigation that around 25% of children have asthmatic complaints (wheezing). Around 12% of all children have serious complaints (asthma). However the differences between countries are great. Asthma is also prevalent among adults. A significant proportion of people with asthmatic complaints also have allergies. People with asthma or an allergy benefit from a clean and healthy environment free from irritating substances in the air. A healthy and clean environment, free (as far as possible) from allergens (house mites, cat allergens, pollen etc.) and other irritating substances (consider for example chemical vapours or airborne dust) noticeably reduces the chance of asthma

attacks. Exposing children with asthma and other airway disorders to irritants as little as possible reduces the development of the disease, which benefits their daily functioning and future health. Due to the high percentage of pupils with asthmatic problems and the number of sensitive teachers/lecturers, every classroom and school ought, in fact, should be 'low-allergen'. 2.2.2 Learning performance and productivity Various studies have demonstrated that pupils/students learn and perform less well in classrooms with a poor indoor environment. We shall now consider some prominent studies. Ventilation In recent years, various studies have been carried out into the relationship between ventilation and learning performance (Bakó-Biró et al., 2008; De Gids et al., 2006; Shaughnessy et al., 2006; Wargocki et al., 2005). The most recent studies are summarised in Figure 2.2. From this we see that learning performance strongly decreases below a ventilation rate of 4 l/s person. Above 10 l/s person the increase in learning levels off somewhat, but data for these high ventilation rates are limited. In their literature study of research conducted in past years, Mendell and Heath (2005) also conclude that low ventilation rates lead to reduced learning performance.

7

REHVA Indoor Environment and Energy Efficiency in Schools Guidebook – Part 1

130 120 R2 = 0,7305

RP [-]

110 100

Shaugnessy, reading test Shaugnessy, math test

90

De Gids, reading test

80

De Gids, math test Wargocki, math test

70 0

2

4

6

8

10

Ventilation rate [dm3/s pers]

12

Bakó-Biró, time measures 14 Bakó-Biró, Picture recall memory Bakó-Biró, Word recognition

Figure 2.2 Relationship between the ventilation rate and learning relative performance (RP). (Franchimon et al., 2009; Jacobs et al., 2007).

Daylight

Noise

Daylight appears to play an important role in learning performance. The exact relationship is however not known. The influence of daylight on learning performance was investigated by Heschong. In 2002, Heschong studied the learning performance of students in the western US. Based on this study, it may be concluded that there is a connection between the presence of daylight and student performance. However, the study does not explain why such an effect would occur. In 2003, Heschong investigated the influence of daylight on the learning performance of pupils in 480 schools in California, USA. Among other things, this study showed that glare has a negative influence on learning performance. No connection with daylight was found in this study.

Several studies have been performed aiming to show the effects on children of noise and bad acoustics in classrooms (Shield and Dockrell, 2003). Many of these studies are focused on showing a relationship between outdoor noise and school performance, but a lot of evidence has been found concerning a bad indoor acoustic environment and scores on normalized tests.

8

Main effects on performance due to high outdoor noise can be summarized as follows: the higher the outdoor noise levels, the lower the long term memory performances, the reading abilities, concentration in general, and the scores on SAT (Standardized Assessment Test).

2 INDOOR ENVIRONMENTAL COMFORT ASPECTS

Studies based on reading performance tests, word intelligibility tests, numbers, letter and word identification and SATs have shown an increase of performance of students in classrooms acoustically treated with respect to others staying in un treated ones. Thermal comfort Various studies show that temperature affects productivity. However, these studies were conducted in office buildings rather than educational buildings. It may be assumed that there is also a relationship between temperature and productivity loss in school personnel.

RP [-]

One important study is the meta-analysis (analysis based on the research of third parties) by Seppänen et al (2005) in which the influence of temperature on productivity was investigated (Figure 2.3). This analysis shows that the average productivity reduces by around 2% per degree Celsius once the temperature rises above 25ºC.

1

The risk of spreading infectious diseases is greatest in places where many people are close together. Classrooms are therefore a great risk. Transfer through the air by bioaerosols is an important phenomenon in catching infectious diseases in educational buildings. Bio-aerosols get into the air for example through coughing and sneezing. The role of ventilation in airborne transmission of infectious agents was investigated by groups including Li et al (2007). Based on a systematic review of literature, they concluded that sufficient fresh air supply prevents the transmission of infectious diseases through the air. How much ventilation is needed in schools to restrict this risk cannot be specified based on this study. Shendell et al (2004) demonstrated that ventilation has a positive effect on reducing absence of pupils. The CO2 concentrations and student attendance were registered in over 400 American schools. In schools where the CO2 concentration was 1000 ppm higher than in the outdoor air, absence proved to be 10 to 20% higher. It may be assumed that sick leave of teachers also increases as the CO2 concentration rises. 2.3

0,95 0,9

Thermal comfort

Thermal comfort is defined as that state of mind in which someone expresses satisfaction with respect to thermal environment (ASHRAE, 2004).

0,85 0,8 15

2.2.3 Sick leave

20

25 30 Temperature [°C]

35

Figure 2.3 Influence of temperature on relative productivity (RP) in office work. (Seppänen et al., 2005).

Thermal comfort is realised when both general thermal comfort (related to the body as a whole) is reached and local thermal discomfort (related to thermal exchange in particular body areas) is avoided. 9

REHVA Indoor Environment and Energy Efficiency in Schools Guidebook – Part 1

General thermal comfort is strictly related to thermal neutrality of the human body, which is assured by the thermoregulation system which aims to maintain a constant internal body temperature inside a limited range of values. The internal body temperature has to keep steady at about 37°C, while the external one may vary between a maximum of 45°C and a minimum value depending on the body part (i.e. hands, feet, head etc.). The human body is a thermodynamic system that generates thermal energy and interacts with the external environment. A heat balance equation can be written for human body, as follows:

S = M – W – E – Eres – Cres – C – R – K (2.1) where: S = body heat storage rate, W; M = metabolic rate, W; W = effective mechanical power, W; E = evaporative heat flow at the skin, W; Eres = respiratory evaporative heat flow heat loss, W; Cres = respiratory convective heat flow, W; C = convective heat flow, W; R = radiative heat flow, W; K = conductive heat flow, W. The concept of Equation 2.1 is schematically reported in Figure 2.4. Usually terms contained in (2.1) are referred to the unit of body surface area, calculated from the equation:

ADu= 0,202 Wb0,425 Hb0,725

10

(2.2)

where: ADu = Du Bois body surface area, m²; Wb = body mass, kg; Hb = body height, m;

Figure 2.4 The heat balance of the human body.

S is the consequence of activity and heat exchanges within the environment. When someone occupies an environment, after some time S resets to zero, because, thanks to the thermoregulation mechanisms, the balance terms equilibrate each other. To avoid danger to human life, S should be zero for internal body temperatures within the range 35–38°C. It should be noticed here that, when the condition of S equals to zero is reached, for internal temperatures within 35°C and 38°C, it can be possible that the person is not in a thermal comfort state. Metabolism is a complexity of mechanisms that make chemical energy, due to swallowed food and drink, change into thermal energy inside human body. Generally, metabolic energy is referred to the time and area unit (W/m²); it is usual referring to metabolic rate using the met unit, defined as follows:

1 met = 58,2 W/m²

(2.3)

2 INDOOR ENVIRONMENTAL COMFORT ASPECTS

Metabolic rate can be divided into two parts: one, called basal metabolism, which is necessary to keep organs alive and is measured in mentally and physically restored people; the other part is related to activity of the subject. Estimation of metabolic rate can be performed by means of tables or by direct measurement. Estimation and measurement methods are described in the EN 8996 Standard (CEN 2004). Effective mechanical power represents that part of the energy exchanged by the human body at work with the environment. Usually W is assumed to be null, because it is a very low value in the context of the balance equation. If we consider the yield η of human body as the ratio between mechanical power and metabolic rate, then the difference (M-W) in (2.1) can be expressed as a function of metabolic rate, as follows:

(M – W) = M (1 – η)

(2.4)

Evaporative heat flow from the skin is composed of two parts: heat loss by evaporation due to the diffusion, Ed (independent from sweating) and heat loss by evaporation due to sweating, Esw:

Ed = λ μ (psk – pa)

(2.5)

where: λ = latent heat of water at skin temperature, J/kg; μ = permeance coefficient of the skin, kg/(s m² Pa); psk = water vapour pressure at skin temperature, Pa; pa = water vapour partial pressure, Pa.

Once sweat is produced by sweat glands, it spreads over the skin and evaporates. This is expressed by:

Esw = 0,42 [(M – W)– 58,2] (2.6) Eres and Cres are the two parts of respiratory heat flow and related to the difference in terms of humidity ratio and in terms of temperature between expired air and environmental air respectively. The term Eres is calculated from:

Eres = λ m a,res (Wa,ex – Wa) (2.7) where: m a,res = pulmonary ventilation, kgdry air/s; Wa,ex = humidity ratio of expired air, kgwater/kgdry air; Wa = humidity ratio of air in the environment, kgwater/kgdry air. Furthermore, the expression for Cres is:

Cres = m a,res cp,a (tex – ta)

(2.8)

where: cp,a = specific heat of dry air at constant pressure, J/(kg K); tex = expired air temperature, °C; ta = air temperature, °C. Following in the analysis of (2.1) terms, C is the convective heat flow and it is expressed by the equation:

C = fcl hc Ab (tcl – ta)

(2.9)

where: fcl = clothing area factor, N.D.; hc = convective heat transfer coefficient clothing-air, W/(m² K); tcl = clothing surface temperature, °C.

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REHVA Indoor Environment and Energy Efficiency in Schools Guidebook – Part 1

tr = ∑ ti ⋅ Fp −i

The term hc is calculated by: in natural convection:

hc = 2,38 tcl − ta

0,25

(2.10)

in forced convection:

hc = 12,1 var

(2.11)

where: var = relative air velocity, m/s. Radiative heat flow, R, depends on several factors expressed by the equation:

R = Aeff · ε · σ [(tcl + 273) – (tr + 273)4] (2.12) where: Aeff = effective radiating area of body 1, m²; ε = emittance of the outer surface of the clothed body, = 0,97, N.D.; σ = Stefan Boltzmann constant, equal to 5,67 10-8 W/(m² K4); tr = mean radiant temperature, °C. Mean radiant temperature is defined as the uniform temperature of an imaginary black enclosure in which an occupant would exchange the same amount of radiant heat as in the actual non-uniform enclosure. Mean radiant temperature can be calculated with the following equation (2.13) 2:

1

Aeff = ADu ⋅ f cl ⋅ f eff

where: ti = temperature of a generic isotherm surface (i.e. wall, other person) viewing the subject, K; Fp-i = view factor between person and surfaces i, N.D. Conduction heat flow is due to the heat exchange of the body with the solids in contact with it, i.e. chair, floor and objects that a person can hold in his hands. Since this factor is difficult to calculate, it is usually controlled by varying the clothing insulation value. The sum of convection and radiation heat flow is called dry heat loss:

H = R + C = f cl ⋅ ADu (hc + hr )(tcl − to ) (2.14) where to, operative temperature, is defined by:

to =

h r t r + h ct a hc + hr

(2.15)

When the occupants are engaged in near sedentary physical activity (with metabolic rates between 1,0 met and 1,3 met) and out of direct sunlight, when the air velocity is low (< 0,2 m/s) or where the difference between the air temperature and the mean radiant temperature is small (< 4°C), the operative temperature can be calculated with sufficient approximation, as the

(a)

where feff is the effective radiation area factor, defined as the ratio of the effective radiation area of the clothed body to the surface area of the clothed body. Its values may vary by sitting to standing person.

12

(2.13)

2

Expression (2.13) gives mean radiant temperature with a good approximation when there is a difference of less than 10°C between the subject and surfaces interacting with it.

2 INDOOR ENVIRONMENTAL COMFORT ASPECTS

arithmetic mean value of air temperature and mean radiant temperature; a weighted mean value can be used depending on relative air velocity, for higher precision or other environments (ASHRAE, 2004). H also depends on clothing insulation. The heat balance equation (2.1) is used to determinate the thermal state of the human body; the most important variables, involved in (2.1) and which influence thermal conditions, and therefore thermal comfort, are: • • • • • •

metabolic rate; clothing insulation; air temperature; mean radiant temperature; relative air velocity; relative humidity.

Several combinations of these parameters can reach a thermal comfort condition. 2.3.1 Global thermal comfort Global thermal sensation of a person is mainly related to the thermal balance of the body as a whole. As mentioned, this balance is influenced by physical activity and clothing, as well as the following environmental parameters: air temperature, mean radiant temperature, air velocity and relative humidity. A necessary condition to obtain overall thermal comfort is that internal energy of human body must not increase or decrease, that is to say that, in the balance equation, the body heat storage should be null; in this condition, the equation (2.1) assumes the following form:

f ( clothing , activity , t a , va , UR, t r , t sk , Esw ) = 0 (2.16)

This equation consists of eight variables: two of which concern the subject (clothing and activity), while four are related to the environment (air temperature, air velocity, air humidity and mean radiant temperature) and two are physiological variables (skin temperature and evaporative heat flow due to sweating, or percentage of skin made wet by sweat). The two physiological variables are not independent, but they are related to the others through complex relationships, so the actual independent variables from which thermal comfort depends are six. This means that, conceptually, there are ∞5 possible combination of described variables for which equation (2.16) is satisfied. Global thermal comfort indices When these factors have been estimated or measured, the thermal sensation for the body as a whole can be predicted by calculating the predicted mean vote, PMV; this calculation is specified in the EN 7730 Standard (CEN 2005a). The PMV is an index, introduced by Fanger (1970), which predicts the mean value of the votes of a large group of persons on the 7-point thermal sensation scale (see Table 2.1), based on the heat balance of the human body. Thermal balance is obtained when the internal heat production in the body is equal to the loss of heat to the environment. Fanger studies were based on experiments conducted in a climatic chamber and involving 1300 students who were interviewed about their thermal sensation during the experiments (see Figure 2.5). So, the PMV index is typically used for assessing thermal comfort/discomfort conditions in air conditioned environments. The use of PMV

13

REHVA Indoor Environment and Energy Efficiency in Schools Guidebook – Part 1

has now been extended to non air conditioned buildings (Fanger and Toftum, 2002). Table 2.1 Seven point sensation scale. VOTE

Corresponding Sensation

+3

Hot

+3

Warm

+1

Slightly warm

0

Neutral

-1

Slightly cool

-2

Cool

-3

Cold

the actual activity of the subject 3. So, all the possible conditions of thermal comfort are defined by the combination of the six independent variables that satisfy equations (2.6), (2.16) and (2.17). Fanger based PMV calculation on the following hypotheses: • thermal sensation is related to thermal load, L, defined as the difference between internal heat production (M – W) and the heat loss to the actual environment for a man hypothetically kept at the comfort values of the mean skin temperature and the sweat secretion at the actual activity level (Fanger, 1970). In the thermal neutrality condition the thermal load will be equal to zero; • the equation that describes the relationship between PMV and thermal load is assumed to be:

Climatic chamber

Figure 2.5 The derivation of PMV index from Fanger’s experiments.

PMV = (0,303 ⋅ e −0,036 M + 0,028) ⋅ L (2.18)

In Fanger’s theory it is underlined that the condition expressed by (2.16) is a necessary one, but insufficient to realize overall thermal comfort; in fact, the following equation should also be used:

Fanger defined another index, PPD (Predicted Percentage of Dissatisfied) that represents the percentage of people voting ±2 or ±3 on the thermal sensation scale (see Table 2.1). The relationship between PMV and PPD, is described by the equation:

Esw = 0,42 [( M − W ) − 58,2]

PPD = 100 − 95 ⋅ e( −0,03353⋅PMV

t sk = 35,7 − 0,0275

(M − W ) Ab

(2.6)

−0,2179⋅PMV 2 )

(2.19) (2.17)

From these last two equations it can be seen that the actual Esw and tsk under thermal comfort conditions are a function of

14

4

which is quoted in Figure 2.6.

3

In the past it was considered that another necessary condition for overall thermal comfort could be the non activation of thermoregulation mechanisms, but it has been shown that these are related to activity, that, raising, can make thermoregulation mechanism start even under thermal comfort conditions.

2 INDOOR ENVIRONMENTAL COMFORT ASPECTS

DR = (34–ta)·(va–0,05)0,62·(0,37·va·Tu +3,14) (2.20)

40 30 20 10 8 6 4

-2

-1.5

-1

-0.5

0

0.5

1

-1,5

2 PMV

Figure 2.6 PPD (Predicted Percentage of Dissatisfied) as a function of PMV (Predicted Mean Vote). (CEN, 2005a).

2.3.2 Local thermal discomfort The PMV and PPD indices are useful for assessing comfort or discomfort condition for the body as a whole. Thermal discomfort can also be due to unwanted cooling or heating of one particular part of the body: this condition is called thermal local discomfort or local discomfort. Local thermal discomfort can be due to: • • • •

where: ta = local air temperature, in °C, supposed to be in the range 20 – 26°C; va = local mean air velocity, m/s, supposed to be less than 0,5 m/s; Tu = turbulence intensity, N.D. Vertical air temperature difference High vertical air temperature difference between head and ankles can cause discomfort. Figure 2.7 shows the percentage of dissatisfied (PD) as a function of the vertical air temperature difference between head and ankles. PD

PPD

80 60

80 60 40 20 10

draught; vertical air temperature differences; warm and cool floors; radiant temperature asymmetry.

6 4 2 1

Local thermal discomfort indices

0

2

4

6

8

10 Δt a,v

Methods for assessing local thermal discomfort factors are itemised as follows.

Figure 2.7 Percentage of dissatisfied (PD) as a function of the difference of temperature between head and ankles. (CEN, 2005a).

Draught

Warm and cool floors

The discomfort due to draught may be expressed as the percentage of people predicted to be dissatisfied with respect to draught (Draught Rate, DR):

If the floor is too warm or too cool, occupants could feel discomfort. Figure 2.8 shows the percentage of dissatisfied as a function of the floor temperature. 15

PD

REHVA Indoor Environment and Energy Efficiency in Schools Guidebook – Part 1

80 60 40 20 10 6 4 2 1

5

10

15

20

25

30

35

40 Δtf

Figure 2.8 Percentage of dissatisfied (PD) as a function of the floor temperature. (CEN, 2005a).

Radiant temperature asymmetry

PD

Radiant temperature asymmetry, ∆tpr, can cause discomfort. Figure 2.9 shows the percentage of dissatisfied as a function of the radiant temperature asymmetry caused by a warm ceiling, a cool wall, a cool ceiling or by a warm wall.

80 60 40

1

2

20

3

4

10 6 4 2 1

0

5

10

15

20

25

30

35 Δtpr

Figure 2.9 Percentage of dissatisfied (PD) as function of radiant temperature asymmetry, °C, for warm ceiling (1), cool wall (2), cool ceiling (3) and warm wall (4). (CEN, 2005a).

2.3.3 Environments classification Environments are classified as a function of thermal environment: three categories, A, B and C are defined by EN 7730 (see

16

Table 2.2). All the criteria should be satisfied simultaneously for each category. It is important to notice here that the EN 7730 Standard categories are not the only ones available for categorization of environments. EN 15251 Standard (CEN, 2007d) also provides categories for thermal environment (see Table 2.2). These last mentioned categories are similar to the ones introduced by EN 7730, except for the name and for the introduction of one more category; this topic will be discussed in detail in the following. 2.3.4 Subjective assessment of thermal comfort When an environment is investigated independently by physical measurements, it could be very important to assess how people occupying that space feel with regards to thermal environment. This is important because subjective response to environment could differ from what the described model predicts, since the model itself is based on a statistical analysis. A special Standard, the EN 10551 (CEN, 2001a), is the reference for subjective thermal comfort assessment of occupants in an environment. Judgement scales are defined aiming to assess: • thermal sensation (perception scale); • satisfaction with regard to the environment (evaluative scale); • need of changing thermal conditions (preference scale); • acceptability of thermal environment (acceptance scale and tolerability). Statistical methods are provided for treating collected data as well.

2 INDOOR ENVIRONMENTAL COMFORT ASPECTS Table 2.2 Categories of thermal environment. EN 7730 Category A

Category B

PPD PD (%) PMV

-0,20 – 0,20

Category C

PPD PD (%)

PPD PD (%)

≤6

-0,50 – 0,50

≤10

-0,70 – 0,70

≤15

ta,1.1-ta,0.1